Mixing in the free atmosphere above the atmospheric boundary layer (ABL) and the deep ocean below the oceanic mixed layer (OML) has been a tough problem to solve since it is episodic and hard to model using second moment closure. The generation mechanisms in these generally stably stratified regions are not as well understood. The current consensus is that the mixing in the deep ocean is mainly internal wave-driven, although in many places double-diffusion is also important. In the free atmosphere, mixing may be driven by mean shear and associated shear instabilities as well as internal wave breaking.
Mixing caused by the jetstream at the tropopause and mountain waves breaking near the tropopause are potentially important in the exchange of trace gases such as ozone and greenhouse gases such as carbon dioxide between the troposphere and the stratosphere. Clear air turbulence (CAT) is also of importance from the point of view of safety and comfort of air travelers. There is significant temporal and spatial variability in the dissipation rate aloft, precluding the use of arbitrary constant values. A dearth of measurements contributes to a lack of knowledge about the characteristics of regions of intense small-scale mixing, which gives rise to questions regarding the role of this mixing in climate and weather prediction.